Method and apparatus for ultrasonic measurement of a physical parameter

ABSTRACT

A parameter such as the depth (d) of a liquid (12) in a tank (10) is measured by use of an ultrasonic transducer (14) outside the tank (10). This is possible by choosing the ultrasonic frequency and the thickness of the tank wall such that a defined relationship is satisfied.

This invention relates to measurement of physical parameters usingultrasonics.

It has hitherto been thought that it is not practicable to measure someparameter, e.g. depth, within a vessel or container by means of anultrasonic transducer external to the vessel or container because of thereflections and resonance produced by the ultrasonic beam interactingwith the vessel or container itself. The present invention provides ameans of doing this, with obvious advantages of simplicity, reliabilityand cost.

The present invention provides apparatus for measuring a physicalparameter on one side of a wall from a location on the opposite side ofthe wall, including an ultrasonic transducer secured to the wall and anoscillator arranged to drive the transducer to emit ultrasonic energy ata given frequency, in which said frequency is such that the thickness ofthe wall satisfies the relationship:

    t=K.sub.o.λ/4

where t is the wall thickness, K_(o) is an odd number 1,3,5 . . . , andλ is the wavelength of the ultrasonic energy in the wall material, forthe case where the acoustic impedance of the wall is lower than that ofthe transducer crystal material; or

    t=K.sub.e.λ/2

where t and λ are as defined above and K_(e) is an even number 0,2,4 . .. , for the case where the acoustic impedance of the wall is equal to orhigher than that of the transducer crystal material.

From another aspect, the invention resides in a method of measuring aphysical parameter on one side of a wall from a location on the oppositeside of the wall, comprising abutting an ultrasonic transducer againstsaid opposite side of the wall and driving the transducer at a frequencyselected such that the wall thickness satisfies the relationship:

    t=K.sub.o.λ/4

where t is the wall thickness, K_(o) is an odd number 1,3,5 . . . , andλ is the wavelength of the ultrasonic energy in the wall material, forthe case where the acoustic impedance of the wall is lower than that ofthe transducer crystal material; or

    t=K.sub.e.λ/2

where t and λ are as defined above and K_(e) is an even number 0,2,4 . .. , for the case where the acoustic impedance of the wall is equal to orhigher than that of the transducer crystal material.

The same transducer may be used as a receiver, or another transducer maybe positioned against the wall as receiver. The measurement of interestis derived by comparing transmitted and received reflected signals, e.g.by timing, by known techniques.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawing which is a schematicillustration of the invention used for measuring liquid level in a tank.

An aluminium tank 10 holds a liquid 12 whose depth d is to be monitored.An ultrasonic transducer 14 of any convenient type is secured againstthe bottom wall of the tank 10. An oscillator 16 operating at afrequency f is connected via gate 18 to the transducer 14 to producepulses of ultrasonic energy frequency f.

The frequency f is chosen in this case to be such that the wall of thetank 10 forms a quarter-wavelength plate. Thus f is determined by thewall thickness and the sonic velocity for the material of the tank. Forexample, an aluminium tank wall of 6 mm thickness will require afrequency of 250 kHz.

This choice of frequency avoids any sizeable echo or resonance from thetank 10. The ultrasonic pulses are thus reflected from the liquidsurface and detected by the transducer 12, allowing d to be determinedby measuring the transit time by timer 20.

The same technique can be used to measure parameters other than depth.For example, the flow speed of a liquid in a pipe could be measured,since the transit time is altered by variations in flow speed.

The above quarter-wavelength relationship is preferred for the casewhere the acoustic impedance of the wall is lower than that of thetransmitter crystal. However, in such case the technique will alsooperate with three-quarters wavelength and higher orders, as defined ingeneral terms by:

    t=K.sub.o.λ/4

where t is the wall thicknss, K_(o) is an odd number 1,3,5 . . . , and λis the wavelength of the ultrasonic energy in the wall material.

If the wall material has an acoustic impedance equal to or higher thanthat of the transmitter crystal, the wall thickness should be one-halfwavelength or a multiple thereof, as defined in general terms by:

    t=K.sub.e.λ/2

where t and λ are as defned above and K_(e) is an even number 0,2,4 . .. .

Lower K_(o) and K_(e) are preferable because of a thinner overall"sandwich" between the transducer face and liquid in the tank thereforereducing the amount of a transmitter reverberation which increases theminimum liquid level that can be accurately measured.

The use of K_(e) =0 is practicable only with low ultrasonic frequencies.

In a modification (not shown) the wall thickness comprises the actualwall of the tank together with an separate plate inserted between theface of the crystal and the wall; this simplifies the transducerconstruction. For example, the wall thickness may be λ/2 and the plateλ/4 thick, giving an effective t=3λ/4.

I claim:
 1. Apparatus for measuring a variable physical parameter of amaterial on one side of a wall from a location on the opposite side ofthe wall, variation of said physical parameter causing a correspondingvariation in the transit time of an ultrasonic pulse through saidmaterial, said apparatus comprisingan ultrasonic transducer secured tosaid wall; an oscillator arranged to drive the transducer to emitultrasonic energy at a given frequency, in which said frequency is suchthat the relationship

    t=K.sub.o λ/4

is satisfied where t is wall thickness, K_(o) is an odd number 1, 3, 5 .. . , and λ is the wavelength of the ultrasonic energy in the wall, forthe case where the acoustic impedance of the wall is lower than that ofthe transducer material, or

    t=K.sub.e.λ/2

where t and λ are as defined above and K_(e) is an even number 0, 2, 4 .. . , for the case where the acoustic impedance of the wall is equal toor higher than that of the transducer material; and timing means coupledto said transducer and arranged to measure the time interval between thetransmission of a signal and the receipt of a reflected signal. 2.Apparatus according to claim 1, wherein said wall is the wall of a tankand the physical parameter to be measured is the depth of a liquid inthe tank.
 3. Apparatus according to claim 1, wherein the thickness tcomprises the thickness of a wall member extending beyond the transducertogether with the thickness of a separate plate positioned between thetransducer and said wall member.
 4. A method of measuring a variablephysical parameter of a material on one side of a wall from a locationon the opposite side of the wall, variation of said physical parametercausing a corresponding variation in the transit time of an ultrasonicpulse through said material, said method comprisingabutting anultrasonic transducer against said opposite side of said wall; drivingthe transducer to emit ultrasonic energy at a given frequency, saidfrequency being such that the relationship

    t=K.sub.o.λ/4

is satisfied where t is wall thickness, K_(o) is an odd number 1, 3, 5 .. . , and λ is the wavelength of the ultrasonic energy in the wall, forthe case where the acoustic impedance of the wall is lower than that ofthe transducer material, or

    t=K.sub.e.λ/2

where t and λ are as defined above and K_(e) is an even number 0, 2, 4 .. . , for the case where the acoustic impedance of the wall is equal toor higher than that of the transducer material; and measuring the timeinterval between the transmission of a signal and the receipt of areflected signal.